Fluorescent resins that are useful in water based ink jet printing inks are based on both molecular weight and degree of crosslinking of the resin during manufacturing. If the molecular weight is not controlled within defined limits, then the viscosity of the resulting jet ink may be too high to be useful, e.g., greater than 10 cps. It is known that the amount of resin in the jet ink also controls the viscosity of the jet ink. However, in the case of fluorescent jet inks, there are two additional parameters to consider. First, the amount of fluorescent dye in the resin determines the degree of fluorescence that the jet ink will exhibit. At low dye concentration, there is minimal fluorescence. As the dye concentration increases, so does the fluorescent brightness of the jet ink until the dye concentration reaches a point that the dye becomes "self-quenching". That is, the dye molecules become too close together and thus absorb the fluorescent light prior to it escaping from the resin.
Therefore, fluorescent jet inks must balance the amount of fluorescent dye in the resin to achieve maximum fluorescence and the amount of resin in the ink to control both jet ink viscosity and fluorescence. It is known that a viscosity between 1 cps and 10 cps is desired for jet inks.
Thus, the concentration of fluorescent resin that can be used is related to the molecular weight of the resin. The larger the molecule, less resin can be employed in order to maintain low viscosity, and thus lower fluorescence is obtained.
Additionally, resins for jet inks must be totally soluble in the ink formulation. Particle sizes in excess of one micron will cause clogging of a typical jet nozzle. Solubility of the fluorescent resin in a water based jet ink is dependent on the chemical structure of the resin, molecular weight, and degree of crosslinking.
Hill and Walker (J of Polym Sci, 3 (5), 609 (1948)) describe the effect of molecular weight on the properties of polyester resins. Their conclusions were that the chemical structure of the polyester correlates to the solubility of the resin. Bruce (IBM J. Res. Develop., May 1976, p. 258) reported that the jet inks containing polymers exhibited longer separation lengths than nonpolymeric jet inks and that the theoretical model for jet inks showed a dependency on polymer concentration. Bruce also states that the viscous "power law" fluid model was inadequate to explain the behavior of the polymeric jet inks. Rubin and Whatshavsky (Israel J. of Tech, 8 (4), 285 (1970)) also show that polymeric jet inks behave in an unpredictable manner.
Broadhead (U.S. Pat. No. 3,053,783) shows that trimellitic anhydride, when reacted with an aliphatic polyol, produces a water soluble resin. Broadhead claims essentially all alkylene glycols, or all alkylene ether glycols, will yield water soluble resins. Thomas, et. al. (U.S. Pat. No. 4.024,111) discuss the use of trimellitic anhydride to produce highly branched polyesters. Crosslinking of polymer chains is known to cause insolubility of the resin. In fact, the amount of insolubility of the resin relates to the degree of crosslinking present in resins.